eq_clean_data: Tidy and subset earthquake data

Description

This function (eq_clean_data) removes unnecessary columns using the select_ function from the dplyr package (select_).
The columns that are kept in the data are: YEAR, MONTH, DAY, LATITUDE, LONGITUDE, LOCATION_NAME, TOTAL_DEATHS, COUNTRY, EQ_PRIMARY.
Earthquakes with negative year values are excluded from the data using the filter_ function from the dplyr package (filter_).
The mutate_ function from the dplyr package (mutate_) is used to create a date column and ensure analysis columns are numeric.